1. Field of the Invention
The present invention relates to a vehicle-use tandem electric rotating machine having a common rotating shaft and a plurality of stator-rotor pairs fixed to this rotating shaft.
2. Description of Related Art
It is known to tandem-connect two Randell type rotors in order to double the output power of an electric rotating machine as disclosed, for example, in Japanese Patent Application Laid-open Nos. 1-157251, 5-137295, and 5-308751. Such a tandem-connection makes it possible to provide a vehicle-use generator (alternator) compact in size and capable of generating two different generation voltages that can be controlled individually. It also makes it possible to reduce the manufacturing cost and save the installation space of an electric power generating system compared to the conventional case where two electric rotating machines (generators) are manufactured and installed separately. Such a tandem electric rotating machine (tandem generator) can be advantageously used for supplying a high voltage to a 42V system in addition to supplying a low voltage to a conventional 12V system.
Incidentally, there is a growing demand to make vehicle-use generators compact in size and light in weight. However, the tandem electric rotating machine as described above is likely to increase in its axial length.
It is known that increasing the rotational speed of a generator is the most effective way to reduce the size of the generator. Increasing the rotational speed of a generator driven by a vehicle engine can be easily attained by reducing the diameter of its pulley (generator pulley). However reducing the diameter of the pulley causes a problem of reduction of generation power, and other problems due to belt slippage especially in a low rotational speed range within which the generator driving torque (generator load torque) seen from the engine side is large. It may occur to increase the diameter of a pulley on the side of a crank shaft of the engine to avoid such problems. However, increasing the diameter of the pulley on the crank shaft side is not easy, because the engine has to drive auxiliaries other than the generator.